1. Field of the Invention
The present invention relates to a sonar transmitter which is used underwater or on the water and, more specifically, to a sonar transmitter comprising a class D power amplifier.
2. Description of the Related Art
General audio power amplifiers can be classified into two types, i.e. linear amplifiers and class D power amplifiers. The linear amplifier is currently the main stream since it can attain high quality by a simple structure though its power conversion efficiency is bad in principle. On the contrary, the class D power amplifier in a pulse-system has advantages over the linear amplifier that it has lower power consumption, can be reduced in size, and has less generation of heat, etc. Therefore, it is expected to put PWM amplifiers into practical use for the future digital sound system.
The operation of a PWM (Pulse Width Modulation) amplifier as typical conventional class D power amplifier is as follows. First, an analog input signal and a triangular wave having a radio frequency of ten times or more of that of the analog input signal is compared by a voltage comparator so as to generate a PWM signal having the duty ratio which is determined in accordance with the amplitude of the analog input signal. The PWM signal switches power amplifying stages constituted with power MOSFET and the like being connected to positive and negative power sources. Then, the output signal of the power amplifying stage is let through a lowpass filter which has a low-resistant inductor as a structural element so as to impede the triangular high-frequency component. Thereby, the audio low-frequency component is extracted. This becomes the output signal of the PWM amplifier and drives a speaker.
Meanwhile, the class D power amplifiers in PWM system are used in sonar transmitters. FIG. 1 is a block diagram for showing a conventional sonar transmitter which uses the class D power amplifier. Explanation will be provided by referring to the drawing.
A conventional sonar transmitter 100 comprises: a load unit 70 having a transformer 73 constituted with a primary wiring 71 and a secondary wiring 72, and a wave transmitter 74 to which a secondary voltage generated in the secondary wiring 72 is applied; a control unit 80 for outputting a PWM signal; and a transmission circuit unit 90 which amplifies the PWM signal outputted from the control unit 80 and applies it to the primary wiring 71 as a primary voltage. The control unit 80 and the transmission circuit unit 90 correspond to the class D power amplifier.
Between the transformer 73 and the wave transmitter 74, a coil 75 is connected for offsetting a capacitance component C of the wave transmitter 74 with an inductance component L. The transformer 73 aligns the output side of the transmission circuit unit 90 with a resistance component R of the wave transmitter 74. The wave transmitter 74 generates a sonic wave 76 in the water.
The transmission circuit unit 90 comprises a power switching circuit 91 composed of a plurality of power switching devices and a drive circuit 92 for driving those power switching devices.
The control unit 80 comprises: a memory circuit 81 formed with a ROM or RAM for storing a sine waveform 87; a feedback circuit 82 which extracts offset voltage from the output voltage of the power switching circuit 91; an adder 83 which adds an inverting component of the offset voltage outputted from the feedback circuit 82 and the sine waveform 87 outputted from the memory circuit 81; and a PWM waveform generating circuit 84 which generates the PWM signal from the result of the comparison between the sine wave outputted from the adder 83 and the triangular wave. The feedback circuit 82 is a lowpass filter, for example. Furthermore, the PWM waveform generating circuit 84 comprises a triangular wave generator 85 for generating the triangular wave and a comparator 86 for comparing the sine wave with the triangular wave.
Re: Japanese Patent Unexamined Publication No. 10-285123
Conventionally, a switching system by a rectangular wave of the transmission frequency(referred to as “transmission frequency switching system” hereinafter) is also used in the sonar transmitter in addition to using the above-described switching system by the PWM waveform (referred to as “PWM system” hereinafter). The transmission frequency switching system uses the rectangular wave with constant amplitude so that it can only change the frequency and the transmission level cannot be controlled. Thus, it cannot be used for transmission such as FM (Frequency Modulation) and the like, which requires a constant frequency band. The reason is that the impedance in the transmitter changes by each frequency so that overcurrent is generated at a frequency by which the impedance becomes low if the transmission level cannot be controlled.
On the other hand, although the conventional PWM system transmission can easily change the transmission level through the duty ratio, the transmission output is decreased compared to the transmission frequency switching system due to deterioration of the voltage use efficiency and an increase in the loss. Specifically, the theoretical voltage use efficiency (effective value) of the sine wave as the transmission waveform in the transmission frequency switching system is 90% as shown in FIG. 2[1], i.e. 4/(Π√2). On the contrary, as shown in FIG. 2[2], it is 71%, i.e. 1/√2 in the PWM system so that the voltage use efficiency deteriorates when using the PWM system. Further, the number of switching is increased in the PWM system compared to the transmission frequency switching system. Therefore, the switching loss is increased thereby decreasing the transmission power, which causes an increase in heat radiation. Thus, a heat radiation mechanism such as a heat sink becomes large-scaled.
Further, as shown in FIG. 1, the PWM system sonar transmitter 100 uses the transformer 73 between the transmission circuit unit 90 and the wave transmitter 74 for achieving the alignment with the resistance component R of the wave transmitter 74. Thus, there generates an offset voltage when switching the power switching circuit 91. There may be cases where the offset voltage excites with the direct current the transformer 73 on the primary wiring 71 side and causes the magnetic saturation thereby damaging the power switching devices. For this, it is necessary to provide the feedback circuit 82 so as not to generate the offset voltage, which complicates the circuit.
Now, described is the reason why the offset voltage is generated when using the transformer. If the primary side of the transformer is a sine waveform, the area on + side and the area on − side of the sine waveform become equal, so that the + flux line amount and − flux line amount in a core (iron core) become equal as well. Meanwhile, if the primary side of the transformer is the PWM waveform, the area on + side and the area on − side of the PWM waveform become slightly different since an error is contained at the time of modulation according to a comparison between the sine wave and the triangular wave. As a result, the offset voltage is generated on the primary side of the transformer. The offset voltage is the direct current so that the power switching devices may be damaged when a large amount of current is flown to the primary side of the transformer. For aligning the high-impedance of the wave transmitter with the low-impedance of the power amplifier, the transformer becomes essential.